OBJECTIVE: The objective of the study was to compare sonographic fetal weight estimates with newborn weight charts and analyze the predictive accuracy of the ponderal index (PI) in preterm infants. STUDY DESIGN: We generated sonographic reference curves for fetal weight and PI estimates from a database of fetal biometric records from 12,589 term deliveries. We then plotted sonographic and newborn weight and PI of 2406 preterm newborns on these curves and compared them with published newborn weight charts. RESULTS: The third centiles of sonographic and newborn weights diverged markedly between 25 and 36 weeks of gestation and by more than 400 g at 32-33 weeks. In contrast, sonographic and newborn PI values were similar despite uncertainties as to fetal length. CONCLUSION: We suggest using sonographic reference fetal weights to screen preterm newborns for low birthweight. Uncertainties in fetal length threaten the reliability of the PI.